This invention relates to a process for tinting or coloring a hydrogel contact lens or other ophthalmic prosthetic device. The process is especially suitable for practice with the lens tinting procedure and lens mounting apparatus described in U.S. Pat. No. 4,518,390 as more fully discussed hereinafter.
Contact lenses can be tinted for cosmetic appearance as well as to reduce light transmission thereby providing the wearer with increased visual comfort. It is, of course, important that the tinted, or colored, lens impart a natural appearance to the wearer's eye. A natural-appearing soft contact lens should have a clear central area, a tinted intermediate area and a clear outer area so that the tinted portion will not cover the sclera of the eye. Moreover, the central portion of the lens should be untinted to permit maximum light passage into the eye interior.
Apart from the foregoing considerations, the nature of the tinting procedure imposes still other requirements if a technically successful and cost-efficient tinting operation is to be realized. It is important that the worker not handle the lens directly with his or her fingers or fingertips because the minute amounts of oil contained on the worker's fingertips will prevent the dye from being accepted or absorbed evenly by the lens. If the worker uses gloves, manipulation of the lens during the tinting procedure can be rendered difficult. If the worker uses tweezers and manually mounts the lens on a fixture, lens production can be slow and accuracy in the mounting may be compromised.
As suggested above, it is important that the tinted annular area of the lens, and correspondingly the clear central portion, be precisely defined and accurately located. Even slight fuzziness or running of dye colors into otherwise uncolored areas of the lens will be immediately apparent. Fuzziness or tint runs will necessitate the rejection of the finished lens product.
By way of achieving a precise and economical procedure for tinting a contact lens meeting the foregoing criteria, U.S. Pat. No. 4,518,390 provides a lens tinting method employing a lens mounting apparatus comprising a tube having a body portion and a closed end portion defined partially by a lens masking structure in communication with the tube body. Suitably treated water, e.g., physiological saline, is placed in the tube to form a column of liquid. The lens to be tinted is dropped into this liquid column and as it falls through the liquid, attains a state of dynamic equilibrium, with the lens convex side down. Further, the hydraulic forces acting on the lens as it passes through the column of liquid will cause the lens to center itself with respect to the interior of the tube body. Thus, as the lens reaches the bottom of the tube, it will be centered as it comes to rest upon the lens mask structure formed at the tube bottom. In one embodiment, this mask structure includes two coaxial seats defining an open annular lens area therebetween. It is this annular area which is to be tinted.
When the lens is centered on the mask, a mandrel device is inserted into the tube. This mandrel has a nose which substantially conforms to the concave surface of the lens. Thus, when the mandrel engages the lens concave side, the convex side of the lens is pushed into firm engagement with the mask structure. The mandrel is then locked within, and upon, the tube. As the mandrel passes through the liquid column it also will be centered due to hydraulic forces acting thereon which produce a state of dynamic equilibrium. Thus, upon engaging the lens, the mandrel is also centered relative to the tube body.
Thereafter, the fixture is inverted and the liquid column in the tube drained away. Next, a tinting medium or dye solution is introduced into a chamber formed in the tube bottom proximate the mask structure. This tinting medium is exposed only to the unmasked portion of the lens and will permeate the lens material of the unmasked portion to provide the desired tint.
The foregoing lens-tinting procedure and apparatus employed therein provides generally excellent results with lenses to be worn by individuals having light colored eyes. For example, a gray iris can be made to take on the color of almost any tinted lens. However, for the estimated seventy percent of the world's population with dark irises, i.e., brown eyes, a change in color is obtained only with the darkest of lens tints as the underlying color of the iris tends to dominate the overall color effect. Such eyes can only be made to appear darker, not lighter.